FWIW Lyman 66's were made in different configurations to suit a lot of flat side receivers. For a 71 you need one that suits the 86 (same width receiver), but current production is made from aluminum alloy whereas older models were made from steel.
Other alternatives are the Lyman 56 and Redfield also made aperture sights to suit the wide 71/86 receivers, but they are hard to identify as they had no markings. You really need to see them along side your rifle to make sure its wide enough.
If you aren't going to change loads and stick to one load/setting the XS peep sights are worth considering. They are probably more robust as the aperture arm of any of the side hung peep sights are exposed and can bend.
